diff --git a/snippets/undefined.py b/snippets/undefined.py
new file mode 100644
index 0000000..baeb491
--- /dev/null
+++ b/snippets/undefined.py
@@ -0,0 +1,117 @@
+# Example of an undefined singleton, usable with eval() functions to limit
+# possibility of NameError exceptions.
+#
+# Use with a namespace decorator like :
+#
+# class NoNameErrorNamespace:
+#     def __init__(self, ns):
+#         self.ns = ns
+#
+#     def __getitem__(self, name):
+#         try:
+#             return self.ns[name]
+#         except KeyError:
+#             return Undefined()
+#
+#     def __setitem__(self, name, value):
+#         self.ns[name] = value
+#
+# eval('x - 1')
+
+
+class Undefined:
+    def __str__(self):
+        return 'Undefined'
+
+    def __getattr__(self, name):
+        return self
+
+    def __neq__(self, other):
+        return True
+
+    self = lambda self, *args, **kwargs: self
+    true = lambda *args: True
+    false = lambda *args: False
+
+    __eq__ = self
+    __lt__ = false
+    __gt__ = false
+    __le__ = false
+    __ge__ = false
+
+    # Arithmetic operators
+    for op in ['__add__', '__sub__', '__mul__', '__matmul__', '__truediv__',
+               '__floordiv__', '__mod__', '__divmod__', '__pow__',
+               '__lshift__', '__rshift__', '__and__', '__xor__', '__or__']:
+        locals()[op] = self
+        locals()[op.replace('__', '__r', 1)] = self
+        locals()[op.replace('__', '__i', 1)] = self
+
+    __call__ = self
+    __getitem__ = self
+    __bool__ = false
+    __abs__ = self
+    __neg__ = self
+    __pos__ = self
+    __invert__ = self
+    __contains__ = self
+    __reversed__ = self
+    __length_hint__ = lambda: 0
+    __hash__ = lambda self: id(self)
+
+    __instance = None
+    def __new__(cls, *args, **kwargs):
+        if cls.__instance is None:
+            cls.__instance = super().__new__(cls)
+        return cls.__instance
+
+
+import operator
+import inspect
+import pytest
+
+
+def test():
+    assert Undefined().__lt__(1) is False
+
+    assert Undefined() is Undefined()
+    assert Undefined().x is Undefined()
+    assert Undefined()['a'] is Undefined()
+    assert Undefined()() is Undefined()
+    assert Undefined().x('a', 1, x=3) is Undefined()
+    assert not Undefined()
+    assert not bool(Undefined())
+    assert (Undefined() - 1) is Undefined()
+    assert (1 - Undefined()) is Undefined()
+
+
+ops = [op for key, op in operator.__dict__.items() if callable(op) and key.startswith('__')]
+@pytest.mark.parametrize('obj', [1, '', [], (), {}, dict()], ids=['integer', 'string', 'list', 'tuple', 'set', 'dict'])
+@pytest.mark.parametrize('op', ops, ids=[op.__name__ for op in ops])
+def test_op(op, obj):
+    if isinstance(op, type):
+        pytest.skip()
+    name = op.__name__.strip('_').strip('0')
+    if name in ['index', 'concat', 'countOf', 'delitem', 'indexOf', 'setitem', 'attrgetter', 'iconcat']:
+        pytest.skip()
+    args_len = len([p for p in inspect.signature(op).parameters.values() if p.default is p.empty])
+    if args_len == 0:
+        pytest.skip('no arg')
+    result = Undefined()
+    if name in ['lt', 'gt', 'le', 'ge', 'truth', 'is', 'contains']:
+        result = False
+    if name in ['ne', 'not', 'is_not']:
+        result = True
+    if name in ['length_hint']:
+        result = 0
+    result_reversed = result
+    if name in ['imod', 'mod'] and obj == '':
+        result_reversed = ''
+    if args_len == 1:
+        assert op(Undefined()) is result, f'failed with {op}'
+    elif args_len == 2:
+        assert op(Undefined(), obj) is result, f'failed with {op}'
+        if name not in ['contains', 'getitem'] or (name == 'ctonains' and obj != '' and hasattr(obj, '__contains__')) or (name == 'getitem' and hasattr(obj, '__getitem__') and obj != '' and obj != [] and obj != () and obj != {}):
+            assert op(obj, Undefined()) is result_reversed, f'failed with reversed {op}'
+    else:
+        raise NotImplementedError(f'{op} {args_len}')